The invention relates to a self-tapping screw having a symmetrical thread profile and a rounded transition between the flanks and the roots of the thread. Such screws can be summarized by the term "wood screws" and essentially comprise a head and a screw shank in the form of an elongate cone whose end opposite the head (screw tip) tapers more or less to a point. The screw shank is surrounded by a helically circumferential cutting edge which projects radially from the root of the thread, the latter changing, with a more or less defined bend, to the two flanks of the thread. The flanks themselves are provided with a constant pitch from the root of the thread to their crest so that a constant thread angle results between the two flank lines, usually of approximately 60.degree. (Lueger, Lexikon der Technik [Technical Encyclopaedia], Volume 1, Basics of Machine Construction, page 164, right-hand column). Such screws are also used to screw plastics.
In addition, a plurality of other screws are known which differ from the generic wood screw essentially only by the selected constant thread angle, pitch and height of the flanks. The so-called chip board screw serves as an example.
When used in plastics, all known screws have the drawback that the plastic is damaged when the screws are screwed in, for example in that the pilot holes burst open or stress cracks are formed, with relatively high driving torques being required at the same time.